The invention relates to a bending mandrel for a bending device for pipes, comprising a rigid head and, attached thereto by only one end, an assembly of flexible lamellae which are layered one above the other and can be bent towards one another perpendicularly of the plane of the lamellae, accompanied by the sliding of the lamellae.
Bending mandrels of the kind specified are used in bending devices for pipes, to support the pipe from inside during the bending operation. Such support is required to counteract any kinking of the pipe, corrugation at the inner bending arc, flattenings at the outer arc, oval deformation of the cross-section and any springing back of the pipe after the bending operation. The risk that the aforementioned events may take place with the bending mandrel not inserted is particularly great in the case of pipes having a high external diameter/wall thickness ratio and required to be bent by a comparatively small radius, for example of the order of magnitude of the pipe diameter. Typical wall thicknesses of such pipes are, for example, 1.5 mm with a pipe diameter of 150 mm.
The prior art discloses different kinds of bending mandrels with which it is not possible to obviate all the aforementioned disadvantages.
In a bending mandrel taking the form of a jointed mandrel (DE 197 07 228 A1) a number of solid links are disposed in series on a rigid head and jointably connected to one another. The joints are formed by link surfaces sliding one upon the other, the surface of one link being constructed convex and the surface of the other link concave. It is true that with such a bending mandrel the circular cross-section of the pipe supported thereon is maintained during the bending operation in the zone of each link, due to the solid construction of the links, but this does not take place over the entire axial length of the arc. The reason is that due to the convex (spherical) shape of the links, the pipe is supported only at axially offset narrow annular zones. In the unsupported zones, therefore, the pipe may become undesirably deformed by either flattenings or folding.
Such undesirable deformation of the pipe during bending is intended to be obviated by another known solid inner mandrel (EP 0 856 367 A2). In this case the bending mandrel is made of plastics (polyurethane) and is solid in construction. The disadvantage of such a bending mandrel is on the one hand that the bendability of a solid bending mandrel is inadequate for bending pipes with small radii of bending, while on the other hand for supporting purposes a compromise must be reached between the bendability of the bending mandrel and stability of shape. If high stability of shape is to be ensured, heavy bending forces must be exerted. The bending force must be greater in proportion as the pipe diameter increases. Clearly, for this reason limits are quickly reached in the use of such a mandrel for pipes of large diameter.
Lastly, a bending mandrel of the kind specified is known (JP 619 8349) in which an assembly of flexible plastics lamellae layered one above the other is disposed between outer spring steel plates bearing against the inside of the pipe. Since the individual lamellae can slide on one another the bending force to be exerted for the bending operation is lower in comparison with the other bending mandrel of solid plastics material. Nevertheless, the bending force to be exerted is undesirably high, more particularly if pipes of large cross-section and small bending radius are to be bent.
Starting from this prior art, it is an object of the invention to provide a bending mandrel with whose use the pipe is bent without excessive bending force and without flattenings or other undesirable deformations.
This problem is solved according to the invention in a bending mandrel of the kind specified by the feature that except for the lamellae disposed on the outside in the assembly, each of the sheet metal lamellae has a central window-like cutaway portion which so extends longitudinally of the assembly that the lamellae partially register via their narrow edge zones, forming a hollow member.
Although the bending mandrel according to the invention consists of lamellae, due to the selected material (sheet metal) it is stable in shape and can be relatively readily bent, since the central lamellae do not extend over the entire cross-section.
Preferably for the sheet metal of the lamellae a wear-resistant material is selected which has a high modulus of elasticity, a high strength and a low coefficient of friction in relation to the material of the pipe, which is suitably bronze, but more particularly steel, more particularly spring steel.
The bending mandrel can be constructed in different ways.
In a first embodiment the cutaway portion in the cutaway lamellae are open at their unattached ends. In an alternative embodiment the cutaway portion in the cutaway lamellae are closed at their unattached ends. The closed end can be advantageous for supporting the end which might possibly be particularly heavily loaded in practice.
The bending force required for bending can be further reduced by the feature that at least some lamellae have on their side adjacent the inner bending arc notches extending transversely of their longitudinal direction. The bending force can also be reduced by the feature that a sliding means is provided between the individual lamellae sliding on one another.
To facilitate the introduction of the bending mandrel into a pipe, according to a possible feature of the invention the free end of the assembly is rounded off spherically, chamfered at an inclination or converges to a point.
Since the bending mandrel must be introduced into the pipe by its head, it must be adapted to the pipe cross-section. To nevertheless enable the lamellae to be attached as simply as possible to the head, according to a possible feature of the invention on its side adjacent the assembly the head is constructed fork-shaped or comb-like, and a proportion of the lamellae is attached between the tines of the fork or the teeth of the comb, the remaining lamellae being attached to the attached lamellae.
Since during the bending operation the lamellae shift in relation to one another, with the result that the lamella lying on the outer arc is left behind, referred to the plane perpendicular to the pipe axis, in comparison with the lamella lying on the inner arc, support would be lost in that zone unless, according to a further feature of the invention, the lamellae have a stepped differential length such that the lamella on the side with the major bending arc has the greatest length, the lamella on the side with the minor bending arc having the shortest length.
To prevent the springy lamellae from expanding apart, according to another feature of the invention at their free ends the lamellae are held together by a spring attached to the outer lamellae.
As a rule the procedure followed when bending a pipe in a bending device having a bending mandrel is that bending is performed in portions, in which case the bending mandrel must be correspondingly changed in position. In contrast, the invention allows bending in a single operationxe2x80x94i.e., without repositioning the bending mandrel. Accordingly, the invention also relates to a method of bending a pipe, using a bending mandrel according to the invention, in a bending device having a fixed holder for the pipe with the bending mandrel introduced, a supporting member disposed fixed on the side of the minor bending arc, and a guide member for the pipe which can pivot around the centre of the bending arc. Such a method is characterised according to the invention in that the pipe and the bending mandrel used, which extends over the entire length of the bending arc, are axially fixed during the entire bending operation. The method according to the invention is rendered possible since, due to the special construction of the bending mandrel, the bending forces required are relatively low, so that the bending forces to be exerted do not become uncontrollably high, even despite the greater axial length over which the lamellae slide on one another during the bending operation.
If, as in this method, the pipe is retained axially fixed during the bending operation, the effective bending zone moves around the centre of the bending arc during the bending operation. The flexible bending mandrel must therefore be designed with a suitable length. An alternative method according to the invention is effective with a shorter bending mandrel. This method uses a bending device having a fixed holder for the pipe with the bending mandrel introduced, a supporting member disposed on the side of the minor bending arc and more particularly rotatable around the centre of the bending arc, and a guide member for the pipe which can pivot around said centre, Such a method is characterised according to the invention in that the pipe and the bending mandrel used, which extends with its bendable assembly at least over the effective bending zone of the bending arc, are so guided axially freely movable or controlled during the entire bending operation that the bendable assembly of the bending mandrel remains in the effective bending zone.
In this method, to obtain as small a relative movement as possible, the pipe can in a controlled or freely movable manner be adapted, in accordance with its different track speeds on the inner and outer arcs, to the track speeds of the part of the device acting on the pipe. The effective bending zone can be spatially fixed by the axial movement of the pipe, so that the method can be successfully performed with a flexible bending mandrel extending over only a small length of the bending arc.